1. Field of the Invention
The present invention relates to a process for preparing polyolefins. In particular, the present invention relates to a process for preparing polyolefins by the polymerization of an .alpha.-olefin with a catalyst which exhibits an excellent polymerization activity to obtain a polyolefin with a desired controlled molecular weight distribution. Further, it relates to a process for preparing polyolefins, providing polyolefin polymers having good particle form and excellent powder characteristics, and olefin copolymers having a good transparency and excellent melt properties such as melt tension.
2. Description of the Prior Art
As a method for producing polyolefins by polymerization of olefins, it has been known to use a catalyst system comprising a combination of transition metal compounds and organometallic compounds. Furthermore, recently numerous methods have been proposed for preparing polyolefins in the presence of catalyst components, for example, comprising solid catalyst components which mainly consist of magnesium, titanium and halogens using magnesium chloride and titanium tetrachloride, and organometallic compounds as highly active catalysts. However, such catalyst systems which mainly consist of titanium compounds are limited to controlling the molecular weight distribution of the resulting polymer. Accordingly, there has been a demand for catalysts which can control the molecular weight distributions of polyolefins arbitrarily depending on the variation of their quality.
Japanese Patent Publication No. 39714/1977 has already disclosed a method for polymerization which can provide polyolefins having an arbitrary molecular weight distribution by using a catalyst system maintaining a high activity comprising an organometallic compound and a reaction product of metallic magnesium, a hydroxylated organic compound, an organic oxygen compound of a transition metal, a halogen-containing compound of a transition metal and an aluminum halide.
In this Japanese Patent Publication, there is disclosed a method of preparing catalysts having no problems in control of moisture and corrosion of the preparation apparatus used in the preparation of the catalyst component and having many industrial advantages, due to using metallic magnesium which is relatively easily treated compared to magnesium chloride or titanium tetrachloride, hydroxylated organic compounds such as metallic magnesium and alcohols and organic oxygen compounds of transition metals such as titanium tetrabutoxide. However, the catalyst system of this publication is not sufficient in polymerization activity, and consequently, there is still room for improvement in this method.
In addition, in case where an ethylene-.alpha.-olefin copolymer is prepared using the catalyst system of this Japanese Patent Publication, the composition distribution range is broad, and therefore, many .alpha.-olefin units incorporated in the polymer may exist in the low molecular weight side, resulting in stickiness, inferiority in transparency, and other problems in properties.
Furthermore, the polymer particles obtained using this catalyst systems have a small average particle diameters as well as a broad particle size distribution, and consequently the proportion of fine particles in the polymer particles becomes large. Therefore, such polymer particles have insufficient powder characteristics.
On the other hand, Japanese Patent Application Laid-open No. 19309/1983 discloses a process for copolymerization of one or more of C.sub.3 -C.sub.12 .alpha.-olefins with ethylene in the presence of a catalyst composed of a transition metal-containing compound having the general formula: EQU (cyclopentadienyl).sub.2 MeRHal
in which R represents a cyclopentadienyl, a C.sub.1 -C.sub.6 alkyl group or a halogen, Me represents a transition meal, and Hal represents a halogen; and a linear or cyclic aluminoxane having the general formulae: EQU Al.sub.2 OR.sub.4 (Al(R)--O).sub.n
for a linear aluminoxane, and EQU (Al(R)--O).sub.n+7
for a cyclic aluminoxane in which n is a number of 4 to 20 and R represents a methyl group or ethyl group.
In this publication, it is disclosed that the copolymerization of ethylene and .alpha.-olefin carried out in the presence of, for example, methyl aluminoxane and bis(cyclopentadienyl)zirconium dichloride can provide an ethylene .alpha.-olefin copolymers having an arbitrary composition while maintaining a high polymerization activity.
Such catalysts composed of transition metal compounds and aluminoxanes are remarkably superior in polymerization activity and copolymerization activity to the conventional Ziegler-Natta catalyst. However, most of such catalyst systems are soluble in the reaction system. Therefore, in slurry polymerization or gas phase polymerization the bulk density of the polymers obtained by using such catalyst systems is small and inferior in powder characteristics. In view of such problems, there is a demand for a solid catalysts on which the above-mentioned transition metal compounds are fixed.
On the other hand, numerous other methods have been proposed in Japanese Patent Application Laid-open No. 35006/1985, for instance, in which the catalyst system to be used is composed of a solid catalyst component in which said transition metal compound is fixed on a porous inorganic oxide carrier such as silica, silica-alumina, etc. and an aluminoxane. In such methods, however, the polymerization activity becomes remarkably lower. Furthermore, most of the polymers obtained by using such method contain much fine powders, resulting in insufficient powder characteristics, such as bulk density. In addition, there has been a problem that the screen mesh of the extruder may become blocked with silica present in the produced polymers, resulting in lowering of processability.
On the other hand, when said transition metal compounds are fixed on chlorine-containing compounds such as magnesium chloride, there is a possibility of inactivation of said transition metal compounds since the carrier itself is acidic. Furthermore, obviating corrosion of the preparation apparatuses has also been required.